An automatic transmission establishes various power flow paths having different speed ratios by selectively engaging and disengaging a number of shift elements. The shift elements include friction clutches. A controller sets the torque capacity of each friction clutch by sending control signals. For example, a controller may send a pulse width modulated signal to a solenoid such that the electromagnetic force exerted by the solenoid is proportional to the pulse width. The solenoid may be connected to a spool valve in a valve body such that the hydraulic pressure in a particular passageway of the valve body is proportional to the electromagnetic force. Fluid from the passageway may be routed to a piston apply chamber of a clutch causing the piston to exert a force related to the fluid pressure. The piston force may squeeze friction plates between separator plates to establish a torque capacity of a clutch. Alternatively, the signal may influence the torque capacity by other mechanism such as causing a motor to rotate, thus causing a piston to apply force to a clutch pack. Due to the indirect causal chain between the signal and the clutch torque capacity, a number of unpredictable noise factors may influence the relationship.
When the controller determines that a speed ratio change is needed, the controller may execute a shift by releasing one shift element called an off-going element and engaging another shift element called an on-coming element. For the shift to feel smooth to vehicle occupants, it is important that the torque capacity of the on-coming element and the torque capacity of the off-going element be coordinated with respect to one another and with respect to the engine torque. For example, if the off-going clutch is released prematurely, then the driveshaft torque will drop excessively and the engine speed will rise. This phenomenon is called an engine flare. On the other hand, if the off-going element is released too late, then the two shift elements will resist one another and the output torque will drop excessively. This phenomenon is called a tie-up.
In order to adjust the control signals such that the torque capacities are proper in the presence of unknown noise factors, a controller may utilize a feedback signal. For example, a controller may utilize signals from an input speed sensor and an output speed sensor to compute the current speed ratio of the transmission. An unexpected or excessively large increase in the ratio of input speed to output speed may indicate an engine flare. In response to this information, the controller may increase the torque capacity of the on-coming shift element. However, some errors, such as a tie-up, are not necessarily reflected in the speed ratio. Furthermore, it takes time for an error in shift element torque capacity to show up as a change in the speed ratio. Therefore, it is desirable to supplement this information with a measurement of the transmission output torque.